Embodiments of the present technology generally relate to ultrasonic imaging. More particularly, embodiments of the present technology relate to focusing a two-dimensional (“2D”) phased array to perform four-dimensional (“4D”), also known as real-time three-dimensional (“real-time 3D”), ultrasonic imaging.
Ultrasound is sound having a frequency that is higher than a normal person can hear, for example, higher than 20,000 Hz. Ultrasonic imaging utilizes ultrasound waves to create an image. That is, ultrasonic imaging systems transmit ultrasonic sound waves, for example, in the range of 2 to 13 MHz, into a subject, such as a patient, receive echoes that are reflected back from the subject and interpret those echoes, thereby creating an image.
Ultrasonic imaging systems utilize transducers to transmit and detect ultrasound. A transducer is a device that converts a signal from one form to another. Some ultrasonic transducers have more than one, also known as an array, of transducer elements. Such transducers are known as phased array transducers. In phased array transducers, each transducer element can transmit ultrasound waves. Likewise, in phased array transducers, each transducer element can detect echoed ultrasound waves.
A focal zone is an area at which transmitted ultrasound waves are focused. It is desirable to transmit ultrasound waves so as to achieve peak pressure at the focal zone. When using phased array transducers, achieving peak pressure at a focal zone occurs when each transmitted ultrasound wave reaches the focal zone at the same time or in-phase. In order to allow each ultrasound wave transmitted from a phased array transducer to reach the focal zone at the same time or in-phase, the transducer can vary the amplitude and/or phase of the wave transmitted from each transducer element based on the location of the transducer element and the location of the focal zone. This practice is known as beamforming.
Beamforming can also be used in connection with receiving echoed ultrasound waves. That is, waves that are reflected from the focal zone at the same time and which arrive at transducer elements at separate times can be amplified or delayed in separate processing channels and then combined in a beamformer to create the best beam possible. The beam is then used to create an image. Failure to properly beamform can result in large sidelobes and/or large temporal misalignment of pulses, both of which are undesirable because either can result in reduced image quality
Forming a best possible image at all times for different anatomies and patient types is important to diagnostic imaging systems. Poor image quality may prevent reliable analysis of an image. For example, a decrease in image contrast quality may yield an unreliable image that is not usable clinically. Additionally, the advent of real-time imaging systems has increased the importance of generating clear, high quality images.
A 2D phased array can be used to produce 4D ultrasound images. However, a 2D phased array can have several thousand elements, and acquiring and controlling data from this many elements can be difficult and costly.
One type of transducer that has been used with a 2D phased array to produce 4D ultrasound images is a Capacitive Micromachined Ultrasonic Transducer (“cMUT”). CMUT's can convert electrical signals into acoustic signals, such as ultrasonic signals, and can also convert acoustic signals, such as ultrasonic signals, into electrical signals. CMUT's require a direct current (“DC”) bias in order to operate.
As described in the article Elevation Beam Profile Control With Bias Polarity Patterns Applied To Microfabricated Ultrasound Transducers, Chris Daft, Paul Wagner, Satchi Panda and Igal Ladabaum, 2003 IEEE Ultrasonics Symposium-1578, beamforming of a 2D array can be performed in a separable method. For example, beamforming in the azimuthal direction can be performed using delay (and/or phase) and sum, and beamforming in the elevation direction can be performed using cMUT bias control. In such systems, the 2D array can be composed of an array of one-dimensional (“1D”) transducer elements in the azimuth direction with bias functions applied in the elevation direction. The article expands upon those aspects of focusing a 2D phased array to perform 4D ultrasonic imaging and also discusses other aspects of focusing a 2D phased array to perform 4D ultrasonic imaging. However, the approach proposed by the article can result in large sidelobes due to the coarseness of the phase information and large temporal misalignment of pulses, either of which can result in reduced image quality.
Thus, there is a need for improved systems and methods for focusing a 2D phased array to perform 4D ultrasonic imaging, which systems and methods decrease sidelobe size and/or temporal misalignment of pulses, thereby providing improved image quality.